Aqueous sodium ion hybrid batteries with ultra-long cycle life at -50 ℃

Aqueous electrolytes are considered as appealing alternatives for commercial organic electrolytes due to their unique merits, such as inherent safety, low cost and environmental friendliness. Unfortunately, they normally cannot operate under extremely low temperature conditions because of the relatively high freezing point of common aqueous electrolytes. To expand their application ranges, the strong polar solvent formamide (FA) is firstly introduced as a cosolvent for the widely used 17 m NaClO4 electrolyte (m: mol kg−1) in this work. The lowest freezing point of the optimized electrolyte has been significantly reduced below -50 °C. Theoretical calculations and spectroscopic methods reveal that FA has sufficient active sites to coordinate with water molecules, cutting off their direct connections and preventing them from forming long-range order structures under subzero temperatures. When paired with the optimized low-temperature electrolyte, the aqueous sodium ion hybrid batteries (ASIHBs) based on active carbon cathode and organic polymer anode deliver a high capacity of ∼80 mAh g−1 at 1 C (1 C = 150 mA g−1) and 8000 cycles lifespan at 4 C under -50 °C. Encouragingly, the pouch batteries can recharge smart phone with potential application. This electrolyte-modification strategy opens a new avenue to explore superior electrochemical performance for ultra-low temperature ASIHBs.